Any patient with a valid registry card may legally use marijuana for medicinal purposes and their caregiver may assist them in doing so. A patient who is charged with a crime for having more medicine than the law allows may argue in court that possessing this extra medicine was medically necessary. Do not argue with the police while being detained or arrested!
Police are instructed not to harm or neglect any property related to medical marijuana (including plants). Any property seized must be returned as soon as the DA determines possession was for medical use.
Between you and your caregiver, you can legally possess six marijuana plants, only three of which may be mature enough to bear usable marijuana, plus two ounces of marijuana in usable form. If you violate these limits, they can keep you off the registry for one year.
Don't be dangerous (i.e., don't drive under the influence), and don't be obvious (i.e., don't smoke in public or flash your stash). That is still illegal.
If you violate their rules, they'll revoke your card for a year.